the one to which Paulvolio refers to is called a MIRT, and even owning one and not using it is a federal crime. If you wanted to make one though, its relatively easy. It's just an LED strobe. The little receiver on the top of the traffic light senses two different frequencies of strobing, one is low priority and keeps a green light green until you're through the intersection, the other is high priority and switches red lights to green instantly.
Unfortunately, many states are in the process of switching over to a more sophisticated system of mobile traffic signal control.